You cannot use #+INCLUDE inside an export block though (well, you can, but it won't work the way you expect - try it and see!)

[BTW, this assumes a recent version of org-mode. Earlier versions used #+BEGIN_HTML ... #+END_HTML instead.]

However, if you want to modify the footer, you cannot do it with an export block. For that, you have to customize the variable org-html-postamble. You should read the documentation of it with C-h v org-html-postamble RET.